Pi$$ like a racehorse

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Meaning: An urgent need to urinate. Origin: Horses, it turns out, don't always feel comfortable urinating just anywhere. Show horses and racehorses spend a great deal of time in their pens and come to feel safe and secure there. They don't like to urinate outside of those pens and in many cases won't. In fact show and racehorses are frequently returned to their pens to allow them to urinate. Hence racehorses are often walking around outside of their pens with an urgent need to urinate.